The planned experiment in urban firefighters follows up on the findings obtained in a series of studies performed this past year on laboratory animal subjects. Using measures of distortion-product otoacoustic emissions (DPOAEs) at 2f1 -f2, these prior investigations were designed to test the hypothesis that the robustness of the cochlear efferent system was inversely related to the amount of reduction in emitted activity produced by exposure to a standardized octave band of noise (OBN). That is, for both guinea pigs and rabbits, low amounts of efferent-system activity, as indexed with a customized binaural DPOAE test called the efferent sum (ES), were subsequently associated with greater amounts of noise-induced decrements in emission levels. In contrast, high ES scores were related to lesser reductions in DPOAEs following exposure to the OBN. Together, these findings suggest that the ES test is a good predictor of the harmful aftereffects produced by exposure to excessive sounds. The primary aim of the proposed experiment is to test the efficacy of the ES test to predict reductions in DPOAEs, and, thus, in cochlear function, in a group of firefighters as a function of the length of exposure to job-related noises. Although the work will focus on new recruits by monitoring DPOAEs and the ES test over a 1-2 year period, other workers, who have been employed for less than 5 years as firefighters will also be monitored using the identical procedures. This study will provide a unique set of data in a gender and ethnically diverse population of workers that promises to determine the risk of noise-induced hearing loss. Thus, the proposed serially-based investigation has the potential of leading to the development of a superior test that would make a useful addition to hearing-conservation programs as an early detector of occupationally related hearing loss.